While having tremendous potential as a therapeutic tool, clinical use of engineered nanoparticles has also been associated with serious safety concerns. Following systemic injection, nanoparticles interact with blood proteins causing life-threating hypersensitivity. The uptake of nanoformulations loaded with anticancer toxins by immune cells causes' severe immunosuppression and dose-limiting toxicity. Activation of complement cascade is responsible for many side effects and immune uptake of engineered nanomaterials. In the preliminary data, we demonstrate that activation of complement via the alternative pathway is responsible for the majority of uptake of iron oxide nanoparticles by neutrophils, monocytes, lymphocytes, and platelets. Despite the fact that the alternative pathway has been shown to be essential for complement activation in many types of nanoformulations, the strategies to mitigate the alternative pathway activation on nanoparticles are virtually non-existent. The novel contribution of this proposal is to develop nanosurface-conjugated complement inhibitors based on natural inhibitor proteins. These proteins have been used in the therapeutics of complement-related disorders but have never been evaluated for protecting nanosurfaces against complement. Our preliminary data strongly support the hypothesis that conjugation of the natural alternative pathway inhibitors will significantly improv hemocompatibility of nanoparticles. We established the following Specific Aims: 1) Design alternative pathway inhibitors in silico for subsequent conjugation to nanosurfaces. We will perform 3-D computer modeling of the complement factors and the inhibitor proteins on nanoparticle surface to identify candidate inhibitors and conjugation strategies; 2) determine the complement inhibition efficiency of surface conjugated inhibitors. We will overexpress the inhibitor proteins, or chemically synthesize smaller polypeptides. The inhibitors will be conjugated to various types of nanoparticles via an engineered cysteine group. We will determine the efficiency of the conjugated inhibitors as a function of the inhibitor density, linke type, nanoparticle size, and surface chemistry (charge, presence of targeting antibody and fluorescent dye). These experiments will determine the most efficient inhibitors and conjugation strategies; 3) determine the efficiency of the inhibitors in improving hemocompatibility of drug delivery nanoplatforms. We will prepare nanoplatforms loaded with chemotherapy drugs. The nanoparticles will be modified with inhibitors and tested for complement activation and immune cell uptake using blood from healthy individuals and cancer patients. The results will be highly beneficial in guiding future preclinical and clinical development of inhibitor-decorated nanoplatforms.